PROJECT SUMMARY/ABSTRACT Proper regulation of gene expression is required for determining and maintaining cellular identity and for responding to the extracellular environment. In eukaryotes, this entails a multi-step pathway for mRNA from transcription to translation and eventual degradation, and cellular responses to changing conditions often induce highly coordinated changes in many of these steps. The response to adverse stress conditions, such as lack of nutrients or oxidative stress, is often necessary in a wide diversity of organisms in order to ensure cell survival. Given its large energy requirements, changes in protein translation play a particularly critical role in cellular stress responses, resulting in altered translation of thousands of genes. This response also becomes misregulated in the pathologies of a number of diseases. Notably, the ability to circumvent translation regulation during stress is a hallmark of cancer progression that promotes continued cell growth, and alterations to stress responses and the translation machinery also contribute to aging. Despite these disease links, how these massive changes to translation are regulated is not well understood. Ded1 is a conserved RNA helicase that plays critical roles in translation initiation. Alterations in the human homolog of Ded1, DDX3, have been found in a number of cancers, including the pediatric brain cancer medulloblastoma and natural killer/T-cell lymphoma. Mutations of DDX3 are also linked to a cognitive developmental disorder, and DDX3 is involved in replication of several viruses, including HIV. These findings underscore the importance of understanding the normal functioning of Ded1/DDX3 since this can also shed light on its disease-associated functioning. In steady-state conditions, Ded1 stimulates translation initiation; however, recent research has revealed that Ded1 has a major role in the repression of translation during stress conditions, specifically when the TOR pathway, the central nutrient-sensor of the cell, is inactivated. This proposal explores this function of Ded1 and associated factors in controlling the translational response to cellular stress. Specifically, Aim I will characterize the stress function of Ded1 in response to TOR pathway inactivation. The mechanism of this role, which involves remodeling and degradation of the critical translation scaffolding factor eIF4G, will be defined, and the downstream consequences of this mechanism on translation of specific mRNAs will be determined. Upstream regulators and accessory factors for Ded1 and eIF4G during stress will also be identified and characterized. Aim II will then examine Ded1 involvement in another part of the stress response, the formation of stress granules, cytoplasmic accumulations of mRNA and associated proteins. It will also test whether the stress response is affected in cells containing Ded1/DDX3 mutations associated with medulloblastoma. This work will greatly enhance our understanding of the translational response to cellular stress and will inform future studies of its misregulation in cancer and other diseases.